SUBSURFACE RECHARGE SYSTEMS - From the MassHighway Storm Water Handbook for Highways and Bridges

SUBSURFACE RECHARGE SYSTEMS - From the MassHighway Storm Water Handbook for Highways and Bridges

Massachusetts Nonpoint Source Pollution Management SUBSURFACE RECHARGE SYSTEMS GENERAL INFORMATION Applicable DEP Stormwater Management Policy Performance Standards Standards #3 and #4. If sufficient additional storage and appropriate outlet structures are provided, recharge basins may also be used to meet Standard #2. DEP Credit: Construction: Moderate to high Maintenance: High 80% Estimated Range from Literature: >90% Description: Subsurface recharge systems may include trenches, beds, galleys, or dry wells. Such systems have sufficient storage capacity so as to permit the gradual infiltration of runoff.

Pollutant removal is provided by filtration through the soil matrix. Pre-treatment is required to prevent failure of infiltration systems due to sediment accumulation.

Subsurface systems (other than leaching catch basins or leaching basins will rarely be used in the highway setting. These systems have historically had significant failure rates, and site constraints often limit the effective use of infiltration. Recharge BMPs should generally be designed as off-line systems. Separate Design Criteria tables follow this table, for Recharge Trenches and Beds and Recharge Dry Wells and Galleys. • Depth to bedrock or other impermeable substratum • Depth to groundwater • Soils • Slope • Recharge systems can provide high levels of treatment of other pollutants, in addition to TSS removal • High failure rates (particularly without sufficient pre-treatment); replacement/rehabilitation (with a cost about equal to initial construction) may be required • Frequent maintenance may be required • Inspect at least twice annually • Regular sediment removal from pre-treatment systems to prevent clogging • Rehabilitation in the event of failure due to clogging • Periodic removal of debris/trash from flow control structures MA-DEP Stormwater Technical Handbook: http://www.mass.gov/dep/brp/stormwtr/stormpub.htm Schueler, 1987 Young, et.

al., 1996 TSS Removal TSS Removal Potential Constraints to Use Other Considerations Maintenance Requirements Primary Design References Subsurface Recharge Systems pg.1 From the MassHighway Storm Water Handbook for Highways and Bridges http://www.mass.gov/dep/water/wastewater/stormwat.htm

Massachusetts Nonpoint Source Pollution Management Recharge Trenches And Beds DESIGN CRITERIA < 2.0 ha (5 acres) Contributing Drainage Area DESIGN PARAMETER CRITERIA1 Varies with system. Recharge systems are sized for recharging an annual volume, not an event storm. Storm frequency for design flow Compute annual recharge volume using methodology specified in MA-DEP Stormwater Technical Handbook (see Primary Design References), or use an alternative method conforming to accepted engineering practice. Annual Recharge Volume Compute storage volume using methodology specified in MA-DEP Stormwater Technical Handbook.

Required Storage Volume Determine recharge rate based on soil texture/hydrologic group as specified in MA-DEP Stormwater Technical Handbook, confirmed by on-site field testing; or use an alternative method conforming to accepted engineering practice. Design Recharge Rate A safety factor is recommended to allow for the potential clogging of underground systems. Design Safety Factor The system should be designed to drain the design storage volume in 48 hours or less, using the design recharge rate times the applicable safety factor.

Maximum Draw-down Time Depth of system shall be equal to or less than the depth permitting draw-down in the required time.

Maximum Depth of System When the void space in crushed stone is used for storage, the speci- fied stone should be uniformly sized. A porosity (volume of voids divided by total volume of bed) of 0.39 or less should be used for design. Stone Void Space The crushed stone material must be isolated from adjacent in-situ soils by a geotextile fabric designed to prevent the migration of fine soil particles into the void spaces in the stone. Geotextile materials must meet applicable standard specifications, and must be selected based on an analysis of on-site soils conditions. Geotextile An underground system should be designed as an“off-line”system, or otherwise provided with an overflow or by-pass to safely convey flows that exceed the system capacity.

Provisions for Overflow or Bypass Minimum 0.6 meters (2 feet) below bottom of system, unless engineering analysis demonstrates that lesser separation is feasible. Depth to Bedrock or Impermeable Stratum Small systems: Minimum 0.6 meters (2 feet) below bottom of system. Large systems: Groundwater mounding analysis may be required. Depth to seasonal high groundwater Pre-treatment system is required to provide TSS removal prior to discharge to an underground recharge system. Pretreatment Provide measures to dissipate velocity of flows into the device, to prevent erosion within the structure; generally, velocities < 0.61 meters/second (2 fps) are recommended.

Velocity Dissipation at Inlet Subsurface Recharge Systems pg.2

Massachusetts Nonpoint Source Pollution Management 1 Several of the design criteria regarding setbacks from slopes, foundations, and other site features have been adapted from the requirements for on-site sewage disposal systems described in Massachusetts Title 5 (310 CMR 15.000). However, storm water quantities and flow durations differ markedly from the hydraulic loadings to septic systems. The design engineer should be aware of these differences, and may need to consider additional setbacks to provide for slope stability, protect structures, and provide for the satisfactory performance of the recharge system.

Recharge Basin (Continued) DESIGN CRITERIA 3.0 meters (10 feet) Setback from slab foundation DESIGN PARAMETER CRITERIA1 6.1 meters (20 feet) Setback from cellar foundation 4.6 meters (15 feet) (top edge of system to top of slope), or as required for impoundment stability. Distance may need to be greater where potential for“break-out”and resulting slope instability may be a problem. Setback from slope >15% 7.6 meters (25 feet) (or greater, if required under 310 CMR 15.000 [Title 5]) Setback from on-site sewage disposal system 30.5 meters (100 feet) Setback from private well Zone I radius; additional setback may be required depending on hydro-geologic conditions 30.5 meters (100 feet) Zone A, and 30.5 meters (100 feet) from tributaries Setback from surface water supply The infiltration surface shall be constructed to preserve and enhance the capability of the soil to pass flows from the basin into the groundwater.

Consider measures such as minimizing trafficking by heavy construction equipment Construction of infiltration surface If structural chambers are used to construct the bed or trench, they should be designed for dead and live loads appropriate to their location. The minimum design load shall be H-20 loading. Structural design loading for chambers Underground systems should be provided with access ports, man-ways, or observation wells to enable inspection of water levels within the system. At a minimum, provide two (2) ob- servation wells (152.40 mm (6-inch) diameter perforated PVC or HDPE risers) per trench or bed; for beds greater than 372.0 square meters (4,000 square feet) in area, provide one (1) well for each 186.0 square meters (2,000 square feet) (minimum of three wells).

The inspection port should be accessible at-grade (i.e. not buried).

Inspection access Design shall consider accessibility to system, and capability to replace system components, to provide for eventual repair and rehabilitation of the system. Access for maintenance, repair, and rehabilitation Runoff from disturbed areas shall not be discharged to the recharge structure. The contributing site shall be completely stabilized, prior to placing the recharge structure in service. Protection During Construction Recharge trenches and beds should be “off-line” devices, with provisions for the bypassing or overflow of storms exceeding the storage capacity of the trench or bed.

Other Subsurface Recharge Systems pg.3

Massachusetts Nonpoint Source Pollution Management Example of Recharge Trench Subsurface Recharge Systems pg.4

Massachusetts Nonpoint Source Pollution Management Example of Recharge Bed Subsurface Recharge Systems pg.5

Massachusetts Nonpoint Source Pollution Management Recharge Dry Wells And Galleys DESIGN CRITERIA Contributing area will be limited by the size of well or galley used. These devices are typically used for discharging roof top runoff, or small parking areas.

Designer will need to relate size and number of units to the volume of runoff to be treated. Contributing Drainage Area DESIGN PARAMETER CRITERIA2 Varies with system. Recharge systems are sized for recharging an annual volume, not a storm event. Storm frequency for design flow Compute annual recharge volume using methodology specified in MA-DEP Stormwater Technical Handbook (see Primary Design References), or use an alternative method conforming to accepted engineering practice.

Annual Recharge Volume Compute storage volume using methodology specified in MA-DEP Stormwater Technical Handbook. Required Storage Volume Determine recharge rate based on soil texture/hydrologic group as specified in MA-DEP Stormwater Technical Handbook., confirmed by on-site field testing; or use an alternative method conforming to accepted engineering practice. Design Recharge Rate It is recommended that a safety factor be provided to allow for the potential clogging of underground systems. Design Safety Factor The system should be designed to drain the design storage volume in 48 hours or less, using the design recharge rate times the applicable safety factor.

Maximum Draw-down Time Depth of system shall be equal to or less than the depth permitting draw-down in the required time. Maximum Depth of System When the void space in crushed stone is used for storage, the speci- fied stone should be uniformly sized. A porosity (volume of voids divided by total volume of bed) of 0.39 or less should be used for design. Stone Void Space The crushed stone material must be isolated from adjacent in-situ soils by a geotextile fabric designed to prevent the migration of fine soil particles into the void spaces in the stone. Geotextile materials shall meet applicable standard specifications, and must be selected based on an analysis of on-site soils conditions.

Geotextile An underground system should be designed as an“off-line”system, or otherwise provided with an overflow or by-pass to safely convey flows that exceed the system capacity.

Provisions for Overflow or Bypass Minimum 0.6 meter (2 feet) below bottom of system. Depth to Bedrock or Impermeable Stratum Small systems: Minimum 0.6 meter (2 feet) below bottom of system. Large systems: Groundwater mounding analysis may be required. Depth to seasonal high groundwater Pre-treatment of residential roof runoff not required. Pre-treatment of commercial and industrial building roof runoff may need to be considered, on site-specific basis. Pretreatment (roofs) Pre-treatment system required; provide TSS removal rate specified by DEP prior to discharge to an underground recharge system.

Pretreatment (other areas) Subsurface Recharge Systems pg.6

Massachusetts Nonpoint Source Pollution Management 2 Several of the design criteria regarding setbacks from slopes, foundations, and other site features have been adapted from the requirements for on-site sewage disposal systems described in Massachusetts Title 5 (310 CMR 15.000). However, storm water quantities and flow durations differ markedly from the hydraulic loadings to septic systems. The design engineer should be aware of these differences, and may need to consider additional setbacks to provide for slope stability, protect structures, and provide for the satisfactory performance of the recharge system.

DESIGN CRITERIA Provide measures to dissipate velocity of flows into the device, to prevent erosion within the structure; generally, velocities < 0.61 meters/second (2 fps) are recommended. Velocity Dissipation at Inlet DESIGN PARAMETER CRITERIA2 3.0 meters (10 feet) Setback from slab foundation 6.1 meters (20 feet) Setback from cellar foundation 50 feet (top edge of system to top of slope), or as required for impoundment stability. Distance may need to be greater where potential for“break-out”and resulting slope instability may be a problem.

Setback from slope >15% 7.6 meters (25 feet) (or greater, if required under 310 CMR 15.000 [Title 5]) Setback from on-site sewage disposal system 30.5 meters (100 feet) Setback from private well Zone I radius; additional setback may be required depending on hydro-geologic conditions Setback from groundwater supply Zone A, and 30.5 meters (100 feet) from tributaries Setback from surface water supply Structural components should be designed for dead and live loads appropriate to their location.

The minimum design load shall be H-20 loading.

Structural design loading Each well or galley unit should be provided with an access port, man-way, or observation well to enable inspection of water levels within the system. The inspection port should be accessible at-grade (i.e. not buried). Inspection access If inspection port does not provide access, additional manhole access should be provided to each well or galley chamber. Maintenance access Runoff from disturbed areas shall not be discharged to the recharge structure. The contributing site shall be completely stabilized, prior to placing the recharge structure in service.

Protection During Construction Recharge dry wells and galleys should be“off-line”devices, with provisions for bypassing or overflow of storms exceeding the design capacity of the devices.

Other Subsurface Recharge Systems pg.7

Massachusetts Nonpoint Source Pollution Management Example of Recharge Galley Subsurface Recharge Systems pg.8

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